CN105430289A - Method for detecting flicker frequency of LED based on CMOS image sensor - Google Patents

Abstract

The invention discloses a method for detecting the flicker frequency of an LED based on a CMOS image sensor. The method comprises the steps of: obtaining a strip image of a light source area, converting the strip image into a gray image; calculating an average gray value for all pixels in each line of the strip image, supposing that the gray image with bright and dark strips has M lines, storing M average gray values into an array IAverage(n), wherein n is equal to 0, 1, ..., M-1; performing M point DFT operation on the IAverage(n) to work out a frequency response value R(k) of the IAverage(n), wherein R(k) is equal to FORMULA, k is not less than 0 and not more than M-1, and W<nk>M is a Fourier coefficient; detecting K values corresponding to peaks in the R(k) except low frequency components, and setting K values to be KN, which namely is the number of strips in the strip image; dividing total number of lines M by KN to obtain pixel line numbers of bright and dark strip widths, which namely are the regularly varied periodic value of the pixel gray value in each line, setting Row_N to be equal to M/KN; and working out the flicker frequency of the LED fLED which is equal to frow/ROW_N, wherein frow is the line scanning frequency of the CMOS image sensor. According to the method, corresponding relationship between each characteristic value of the image with bright and dark strips and the flicker frequency of the LED is obtained, detection on the flicker frequency of the LED with high reliability is achieved, and good practical application value is provided.

Description

A kind of method detecting LED flicker frequency based on cmos image sensor

Technical field

The present invention, towards LED visible light communication (VLC) positioning field, proposes a kind of method detecting LED flicker frequency based on cmos image sensor.

Background technology

Adopting the mobile phone with cmos image sensor (CIS) to receive LED light signal, by detecting the light and shade striped picture that flicker LED is formed, low rate information transmission can be realized.For making flicker LED form light and shade striped picture on imaging plane, cmos image sensor should adopt belt-type shutter.Prior art 1 is had to propose a kind of visible light communication system, as shown in Figure 1, the LED light source that its transmitter adopts on-off keying (OOK) modulation to drive, receiver adopts cmos image sensor to form light and shade striped picture, carrys out demodulation OOK signal by carrying out image procossing to light and shade striped picture.The system that this paper proposes and coding/decoding method, under short haul connection (tens centimetres) and the environment without the interference of background environment light, can obtain certain communication performance.But if be applied in by the system and method that prior art 1 proposes in actual common room lighting environment, when namely realizing the communication distance of 2-6 rice and there is ambient light interference, its communication performance will become very poor, cannot meet practical application.

Prior art 2 is had to disclose a kind of visible light signal Propagation solution code method.Its basic thought is that transmitting terminal adopts LED light source lamp to glimmer with different frequencies, and receiving terminal adopts cmos image sensor acquisition blinking light to form the striped picture of different light and shade width of fringe.The light and shade width of fringe of striped picture depends on the flicker frequency of LED light source.Transmitter is modulated driving LED light fixture order by frequency displacement on-off keying (FSOOK) and is sent flicker frequency information, the data of each frequency representative some bits.The cmos image sensor of receiving terminal adopts the time at equal intervals to take pictures and obtains some width light and shade striped pictures, then carry out fringe number visual inspection survey to light and shade striped picture.Because different striped numbers represents different flicker frequencies, and then decode binary data; The visible light communication system that prior art 2 proposes as shown in Figure 2.

As shown in Figure 2, the key that visible light communication (VLC) system of modulating based on FSOOK realizes reliable information transfer is decoded to the light and shade striped picture of different in width and obtains LED flicker frequency.The detection striped picture fringe number object method concrete thought that prior art 2 proposes is: first efficient light sources district striped picture is reduced to light-emitting area rectangle further, gray value dualization is carried out to light-emitting area rectangle picture gray value, then to every a line summation of two-value rectangle picture, ask single order or second order local derviation to value again, finally the striped number obtaining picture is processed to the local derviation value of often going.

But verified by actual experiment, this detection picture fringe number object method reliability that prior art 2 proposes is unsatisfactory.If after the distance between cmos image sensor and LED light source (flicker frequency remains unchanged) changes, the striped number in efficient light sources district can change, and can not obtain the corresponding relation between striped number and LED flicker frequency.

Summary of the invention

The present invention, for overcoming at least one defect (deficiency) described in above-mentioned prior art, provides a kind of method detecting LED flicker frequency based on cmos image sensor.The method, by carrying out mathematical modeling to visible light communication system, obtains the corresponding relation between certain characteristic value of light and shade striped picture and LED flicker frequency, and realizes the high LED flicker frequency detection of reliability thus, has good actual application value.

For solving the problems of the technologies described above, technical scheme of the present invention is as follows:

Detect a method for LED flicker frequency based on cmos image sensor, comprising:

S1, acquisition light source region striped picture, convert striped picture to gray scale picture;

S2, calculate average gray values for all pixels of each row of striped picture, assuming that gray scale light and shade striped picture to have M capable, M average gray value is stored in array I _average(n), n=0,1 ..., in M-1;

S3, to I _averagen () carries out M point DFT computing, ask I _averagethe frequency response values R (k) of (n);

Wherein $R\left(k\right)=DFT\&lsqb;I_{Average}\left(n\right)\&rsqb;=\underset{n=0}{\overset{M-1}{\&Sigma;}}{I}_{Average}\left(n\right)W_M^{nk},$ 0≤k≤M-1, for fourier coefficient;

The K value corresponding to peak value (maximum) in S4, detection R (k) except low frequency component, is set to K _n, K _nnamely the striped number in striped picture; Use total line number M divided by K again _nobtain the number of lines of pixels of light and shade width of fringe, namely the periodic quantity of the every Changing Pattern of row grey scale pixel value of striped picture, is set to Row_N=M/K _n;

S5, obtain the flicker frequency f of LED _lED=f _row/ ROW_N; Wherein f _rowfor the line-scanning frequency of cmos image sensor.

The present invention is by carrying out mathematical modeling to the LED lamp of FSOOK modulation driving and the VLC communication system of cmos image sensor combination, the Changing Pattern of the every row grey scale pixel value of known captured light and shade striped picture is periodic, is namely the flicker frequency of LED divided by this periodic quantity by the line-scanning frequency of cmos image sensor.

Compared with prior art, the beneficial effect of technical solution of the present invention is:

The method for light and shade striped image detect LED flicker frequency that the present invention proposes, effectively can improve the reliability of the low rate information transmission of this type of VLC communications localization system, reduce the error rate, improve the ability of anti-various ambient light interference, algorithm is short and sweet simultaneously, significantly can improve the processing speed of light and shade striped picture, there is larger using value.

Accompanying drawing explanation

Fig. 1 is the visible light communication system schematic diagram that prior art 1 proposes.

Fig. 2 is the visible light communication system schematic diagram that prior art 2 proposes.

Fig. 3 is the light and shade bar graph adopting Huawei's smart mobile phone CMOS camera to take pictures formed to LED lamp.

To be CIS adopt belt-type shutter to take pictures imaging process schematic diagram to the LED lamp of flicker to Fig. 4.

Fig. 5 is the amplitude wave-shape figure of the often row pixel average gray value obtained for the striped picture processing of Fig. 1.

Fig. 6 is the specific implementation process flow chart of detection method.

Fig. 7 is the spectral response figure of the often row pixel average gray value obtained for Fig. 3 striped picture processing.

Embodiment

Accompanying drawing, only for exemplary illustration, can not be interpreted as the restriction to this patent; In order to better the present embodiment is described, some parts of accompanying drawing have omission, zoom in or out, and do not represent the size of actual product;

To those skilled in the art, in accompanying drawing, some known features and explanation thereof may be omitted is understandable.Set forth to the imaging process of light and shade striped picture below in conjunction with drawings and Examples.

For light camera communication (OCC) system common at present, transmitter generally can use FSOOK to modulate the LED lamp driven, and can install lampshade additional with controlled light region.Assuming that the flicker frequency of LED lamp is f _lED, flicker cycle is lED lamp sends the square-wave signal that duty ratio is 0.5, then the time-domain expression of LED luminous intensity can be represented by the formula:

E(t)＝E ₀(t+kT ₀)(1)

Wherein:

$E_0\left(t\right)=\left\{\begin{array}{cc}{E}_m,& 0\&le;t<\frac{T_0}{2}\\ 0,& \frac{T_0}{2}\&le;t<T_0\end{array}\right.---\left(2\right)$

Wherein E _mfor luminous intensity peak value.On the other hand, receiver can adopt the CIS of Rolling shutter mechanism.Under the shooting condition of actual scene, in efficient light sources district, light distribution is different, will form specular at center, can reduce gradually toward extension light intensity.Fig. 3 uses the CIS of Huawei smart mobile phone to adopt belt-type shutter to take pictures the instance graph of imaging to glittering LED lamp tool.As seen from the figure, picture can be divided into two levels, and ground floor is the LED lamp picture (specular) that imaging obtains, and also has one deck to be the light and shade striped picture that Rolling shutter is formed.If under the uniform desirable supposed situation of light distribution, the imaging picture in efficient light sources district will only be left the light and shade striped picture formed by Rolling shutter.

Suppose that the line number of the imaging picture of LED lamp is M, often row has N number of pixel.According to the feature of Rolling shutter formula CIS, the time for exposure T of N number of pixel of every a line _ebe the same, and adjacent rows can be separated by when exposing a fixing between-line spacing time T _row.In rolling curtain formula spectrum assignment, keep T interval time between adjacent lines exposure _rownecessary.This is because each pixel often in row needs certain T readout time _pix, and only have when meeting T _row> NT _pixwhen, the light signal of often capable all pixels could be read entirely truely.Fig. 4 gives the schematic diagram that the LED lamp employing belt-type shutter of CIS to flicker takes pictures imaging process.

In the diagram, assuming that carrying out once the new initial time of taking pictures is t ₀, and suppose that the luminous energy loss factor of light in free space and optical system is K _opif, K _rCfor CIS imaging circuit proportionality constant.For the OCC system be made up of a CIS and LED lamp, LED flicker frequency can be pre-set and the time for exposure T of CIS _e.Such as, can by the flicker cycle T of LED lamp ₀be arranged to between-line spacing time T _rowintegral multiple, i.e. T ₀=nT _row, wherein n is positive integer and meets n > 1, and line-scanning frequency is then the i-th line output signal of the pel array of CIS imaging picture can be expressed as:

$\begin{array}{c}V\left(i\right)=V_S\left(i\right)+V_N\left(i\right)\\ =K_{op}{K}_{RC}{\&Integral;}_{t_0+(i-1)T_{row}}^{t_0+(i-1)T_{row}+T_e}E\left(t\right)dt+V_N\left(i\right)\\ =K_{op}{K}_{RC}{\&Integral;}_{t_0+(i-1)T_{row}}^{t_0+(i-1)T_{row}+T_e}{E}_0(t+{\mathrm{knT}}_{row})dt+V_N\left(i\right)\end{array}---\left(3\right)$

Wherein, the Section 1 V of formula (3) _si () represents the i-th row voltage (electric current) signal formed by belt-type shutter shooting imaging, Section 2 V _ni () is a nonnegative value signal, the high light interference noise that the LED lamp captured by expression is formed at the i-th row.Fig. 5 shows the amplitude wave-shape figure of the often row pixel average gray value of stripe pattern shown in Fig. 1, from this figure, walks in 230 row intervals and has larger high light interference value, V (i) signal is raised to eminence 130.

From above formula, work as T _e=T ₀=nT _rowtime, then V _si () signal amplitude size is all identical, show can not produce light and shade striped picture in efficient light sources district in the case.Work as T _e≠ T ₀time, then there is V _s(i)=V _s[i+k (n+1)], now shows the output signal of often going to be the cycle is the periodic waveform of n.Take pictures in process in reality, the time for exposure that CIS chooses need match with the flicker cycle of LED light source, thus in efficient light sources district, form the light and shade striped meeting statistical law.This project is by the mathematical relationship according to LED flicker frequency and light and shade striped, research adopts the signal transacting of specific digital filter realization to striped, thus calculate the novel detection algorithm of LED flicker frequency, and the performance of this algorithm is emulated and surveys checking.

Take pictures in process in reality, the time for exposure that cmos image sensor is chosen is greater than the flicker cycle of LED light source, every a line mean pixel gray value of the light and shade striped picture therefore formed in efficient light sources district relative to " line number " variable be then one by the periodic waveform intercepted.As shown in Figure 3, observation Fig. 3 has 12 light and shade stripeds, then show that this width picture has intercepted the periodic waveform in 12 cycles.

For setting forth the beneficial effect that the present invention has more fully, carrying out simulation analysis below in conjunction with the object lesson shown in Fig. 3, further validity of the present invention and advance being explained.Fig. 6 gives specific embodiment of the invention flow process, is summarized as follows:

1st step: obtain light source region striped picture, an example of striped picture as shown in Figure 3, then converts light and shade striped picture to gray scale picture;

2nd step: the average gray value calculating all grey scale pixel values of every a line, gray value, between 0-255 scope, is stored in an array, forms an average gray value amplitude wave-shape.Such as, the average gray value amplitude wave-shape obtained after processing the striped picture in Fig. 3 as shown in Figure 5;

3rd step: carry out M point DFT to average gray value amplitude wave-shape and convert, wherein M is total line number of striped picture, obtains spectral response value R (k) of often row pixel average gray value.Such as, process Fig. 3 striped picture, the spectral response figure of available often row pixel average gray value as shown in Figure 7;

4th step: detect the K value corresponding to peak value (maximum) except low frequency component in R (k), be set to K _n, then K _nnamely the striped number in striped picture.With total line number M divided by K _nobtain the number of lines of pixels of light and shade width of fringe, namely the periodic quantity of the every Changing Pattern of row grey scale pixel value of striped picture, is set to Row_N=M/K _n;

5th step: the flicker frequency f obtaining LED _lED=f _row/ ROW_N, wherein f _rowfor the line-scanning frequency of cmos image sensor.

Obviously, the above embodiment of the present invention is only for example of the present invention is clearly described, and is not the restriction to embodiments of the present invention.For those of ordinary skill in the field, can also make other changes in different forms on the basis of the above description.Here exhaustive without the need to also giving all execution modes.All any amendments done within the spirit and principles in the present invention, equivalent to replace and improvement etc., within the protection range that all should be included in the claims in the present invention.

Claims (1)

1. detect a method for LED flicker frequency based on cmos image sensor, it is characterized in that, comprise the following steps:

S1, acquisition light source region striped picture, convert striped picture to gray scale picture;

S2, calculate average gray values for all pixels of a line every in striped picture, assuming that gray scale light and shade striped picture to have M capable, M average gray value is stored in array I _average(n), n=0,1 ..., in M-1;

S3, to I _averagen () carries out M point DFT computing, ask I _averagethe frequency response values R (k) of (n);

Wherein $R\left(k\right)=DFT\&lsqb;I_{Average}\left(n\right)\&rsqb;=\underset{n=0}{\overset{M-1}{\&Sigma;}}{I}_{Average}\left(n\right)W_M^{nk},0\&le;k\&le;M-1,$ for fourier coefficient;

The K value corresponding to peak value in S4, detection R (k) except low frequency component, is set to K _n, K _nnamely the striped number in striped picture; Use total line number M divided by K again _nobtain the number of lines of pixels of light and shade width of fringe, namely the periodic quantity of the every Changing Pattern of row grey scale pixel value of striped picture, is set to Row_N=M/K _n;

S5, obtain the flicker frequency f of LED _lED=f _row/ ROW_N; Wherein f _rowfor the line-scanning frequency of cmos image sensor.